Shaking After Training

I trained MMA for the first time in a long time last night, when I got home I was drinking a pint of water and had to hold it with two hands cos I was shaking so much. I know in the past when I have done weight training after a break period, that I've got the shakes while doing that as well.

It's nothing big at all and isn't a worry, I'm just curious as to what the reason behind it is, if any at all? Just wondering if it's a sign of my body needing something, and what that something is?